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Date:   Thu, 8 Jun 2023 14:03:56 -0700
From:   Dan Williams <dan.j.williams@intel.com>
To:     Kai Huang <kai.huang@intel.com>, <linux-kernel@vger.kernel.org>,
        <kvm@vger.kernel.org>
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Subject: RE: [PATCH v11 00/20] TDX host kernel support
Message-ID: <6482423c7c52a_142af8294a6@dwillia2-xfh.jf.intel.com.notmuch>
References: <cover.1685887183.git.kai.huang@intel.com>
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Kai Huang wrote:
> Intel Trusted Domain Extensions (TDX) protects guest VMs from malicious
> host and certain physical attacks.  TDX specs are available in [1].
> 
> This series is the initial support to enable TDX with minimal code to
> allow KVM to create and run TDX guests.  KVM support for TDX is being
> developed separately[2].  A new "userspace inaccessible memfd" approach
> to support TDX private memory is also being developed[3].  The KVM will
> only support the new "userspace inaccessible memfd" as TDX guest memory.

This memfd approach is incompatible with one of the primary ways that
new memory topologies like high-bandwidth-memory and CXL are accessed,
via a device-special-file mapping. There is already precedent for mmap()
to only be used for communicating address value and not CPU accessible
memory. See "Userspace P2PDMA with O_DIRECT NVMe devices" [1].

So before this memfd requirement becomes too baked in to the design I
want to understand if "userspace inaccessible" is the only requirement
so I can look to add that to the device-special-file interface for
"device" / "Soft Reserved" memory like HBM and CXL.

[1]: https://lore.kernel.org/all/20221021174116.7200-1-logang@deltatee.com/